In recent years, attempts have been made to knit a shoe upper using a flat knitting machine. If a thick knitting yarn is used when knitting the shoe upper, the surface of the shoe upper may become uneven, thus affecting the appearance and the wearing comfortableness. If a thin knitting yarn is used for knitting the shoe upper, on the other hand, the surface of the shoe upper becomes smooth and satisfactory appearance is obtained, but the thickness of the shoe upper may not be sufficiently ensured. In this case, the fitting feeling and the strength of the shoe upper may be impaired.
As a solution to the above problem, a technique of knitting the knitted fabric having a double structure can be used, as described in Patent Document 1, for example. In Patent Document 1, a sock and a glove having a double structure are knitted. The knitting procedure of the sock and glove is substantially similar, and the case of knitting the sock will be described below by way of example. First, an inner side knitted fabric portion of the sock having the double structure is knitted from a toe toward a foot insertion opening, and then an outer side knitted fabric portion is knitted from a foot insertion opening toward a toe following the foot insertion opening of the inner side knitted fabric portion (whether to knit the inner side knitted fabric portion or the outer side knitted fabric portion first is arbitrary). As a result, a sock in which the foot insertion openings of the inner side knitted fabric portion and the outer side knitted fabric portion are connected is knitted. When using such sock, the inner side knitted fabric portion is tucked in the interior of the outer side knitted fabric portion to obtain the double structure.